1. Technical Field
The present disclosure relates to a DC-DC converter control circuit to control a DC-DC converter to moderate fluctuation in an output current of the DC-DC converter, and a DC-DC converter including the DC-DC converter control circuit.
2. Description of the Related Art
DC-DC converters that operate to generate a constant output current are known. FIG. 1 is a block diagram illustrating a configuration of a first conventional DC-DC converter. A DC-DC converter 100 shown in FIG. 1 includes transistors TR101 and TR102, diodes D101 through D104, an inductor L101, capacitors C101 and C102, resistors R101 through R107, operational amplifiers OP101 and OP102, a comparator CMP101, an oscillator OSC101, a reference voltage source E101. When an input voltage VIN is input to an input terminal 101, the DC-DC converter outputs an output voltage VOUT via an output terminal 102. The resistor R105 is used for detecting the output current of the DC-DC converter, the operational amplifier OP101 is used as an error comparator to control voltage, and the operational amplifier OP102 is used as an error comparator to control current. The DC-DC converter shown in FIG. 1 detects a voltage difference Vr105 across the resistor R105 generated by current flowing through the resistor R105 and controls the output current so that the output current at the output terminal 102 is kept constant based on the voltage difference Vr105.
In general, in DC-DC converters, an inductor current flowing through the inductor includes a ripple component. It is preferable that the ripple component be within a range of from 10% to 20% of the inductor current (e.g., 100 mA). However, in the DC-DC converter shown in FIG. 1, the strength of the ripple component of an inductor current I101 flowing through the inductor L101 fluctuates in accordance with the input voltage VIN and the output voltage VOUT. As a result, although an average value of the output current can be kept constant for a predetermined time period depending on the operating frequency, the output current itself cannot be kept constant.
FIG. 2 is a block diagram illustrating a configuration of a second conventional DC-DC converter. The DC-DC converter 110 shown in FIG. 2 includes transistors TR111 through TR113, a diode D111, a Zener diode D112, an inductor L111, capacitors C111 through C114, resistors R111 through R117, operational amplifiers OP111 through OP114, an inverter INV111, and a pulse-width modulator PWM111. During rectifying cycle, only during a period during which a voltage difference is present across the terminals of the inductor L111 does the operational amplifier OP111 (integrator) integrate a voltage at a positive terminal to detect the current, and keeps the output current constant based on the signal of the detection result.
However, in the DC-DC converter shown in FIG. 2, the current is detected using the integrator. Therefore, when the inductance of the inductor L111 or the frequency of the pulse-width modulator PWM111 is changed, it is necessary to change the integral constant. In addition, similarly to the DC-DC converter shown in FIG. 2, the strength of the ripple component of the inductor current flowing through the inductor L111 fluctuates, which may not keep the output current itself constant.
Thus, the output current of the DC-DC converter is affected by the fluctuation in the strength of the ripple component of the inductor current. Therefore, in the DC-DC converter, in order to reduce the fluctuation in the output current of the DC-DC converter, it is necessary to keep the strength of the ripple component of the inductor current constant.
In addition, the output current of the DC-DC converter is also affected by the fluctuation in the frequency of the ripple component of the inductor current. Therefore, in the DC-DC converter, in order to reduce the fluctuation in the output current of the DC-DC converter, it is necessary to keep the frequency of the ripple component of the inductor current constant as well.